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Sediment pore‐water profiles were sampled at high resolution (millimeter scale) with a polyacrylamide gel probe. This simple procedure involves inserting a 1‐mm‐thick gel held in a plastic probe into sediment. The gel reaches diffusive equilibrium in <1–2 h. For anions, the gel was sectioned, back‐equilibrated into distilled‐deionized water, and anions determined by high‐performance liquid chromatography. Laboratory trials showed recovery of 104±4% Cl, 102±2% NO3, 101±1% SO4, and 102±2% NH4. For Fe and Mn, the gel was fixed in 0.01 M NaOH for ∼3 h, subsectioned, extracted with 1 M HNO3, and analyzed by atomic absorption spectrometry. Field trials were undertaken in Esthwaite Water, a seasonally anoxic lake in the English Lake District. Gel probe data compared well with conventional pore‐water extractions.

Laboratory incubations with river-bed sediment collected from riffles and pools were used to quantify potential pathways of dissimilatory nitrate reduction in the hyporheic zone of a groundwater-fed river. Sediments collected from between 5-cm and 86-cm depth in the bed of the River Leith, Cumbria, United Kingdom, were incubated with a suite of 15 N-labeled substrates ( 15 NO { 3 , 15 NH z 4 , and 14 NO { 3 ) to quantify nitrate reduction via denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and anaerobic ammonium oxidation (anammox). Denitrification was the dominant pathway of dissimilatory nitrate reduction in the hyporheic sediments, although recovery of 15 N from the ammonium pool indicated that DNRA was also active. The potential for anammox was confirmed by the production of 29 N 2 during the 15 NH z 4 and 14 NO { 3 incubation, but it was much smaller than denitrification. Potential rates of denitrification were highest in shallow sediments and decayed exponentially with depth thereafter. There were clear differences in denitrification activity between riffle and pool sediments. After the production of 15 N-N 2 had stabilized, we added a spike of bacteriological peptone to determine the effect of complex organic substrates on denitrification potential. The potential rate of denitrification increased uniformly at all sediment depths but the total amount of denitrification fueled by the organic substrates decreased markedly with depth, from 90% in the shallow sediments to 30% in the deepest sediments. In addition, a considerable fraction of the 15 NO { 3 could not be accounted for, which suggested that up to 87% of it had been assimilated in the deepest sediments.

A long-term incubation experiment was established to assess the solid Ð solution equilibria of (Cu 2þ ), (Pb 2þ ) and (Ni 2þ ) in soil pore water. The experiment comprised 23 soils spiked with 135, 300 and 75 mg kg À1 of Cu, Pb and Ni, respectively, added as nitrate salts in solution. Samples of soil pore water were extracted several times during the incubation period of 818 days and concentrations of Cu, Pb, Ni, dissolved organic carbon and major cations and anions were measured. Similar measurements were carried out on a smaller selection of historically contaminated soils to extend the range of data and assess compatibility of the measurements with the incubated soils. The chemical speciation program 'WHAM VI' was used to speciate metal ions in solution. A pH-dependent Freundlich equation was used to describe free ion activities, p(M 2þ ), for Cu, Pb and Ni using total adsorbed metal, soil pH, soil organic carbon content and ionic strength as determining variables. For all three metals the greatest improvements in model fit were found when metal ions were assumed to be adsorbed exclusively on soil organic carbon rather than on the 'whole soil'. Further improvements in the description of p(Cu 2þ ) and p(Ni 2þ ) were found when the ionic strength of the soil pore water was included within the model formulation. Residual standard deviations (-log 10 (M 2þ ) units) for the best-fit models were 0.36, 0.53 and 0.29 for p(Cu 2þ ), p(Pb 2þ ) and p(Ni 2þ ), respectively. The effects of progressive fixation of Pb, Cu and Ni on model parameterization, during the course of the experiment, were found to be small. Independent datasets from both published and unpublished sources were used to compare experimental protocols and validate the model for the determination of (M 2þ ) in soil pore water.